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A project log for SAP 1 Computer on Printed Circuit Boards

Inspired by the video series by Ben Eater, I am developing a set of printed circuit boards for the simple as possible computer.

tomtibbettstomtibbetts 07/07/2020 at 22:231 Comment

Hi All,

Sorry for the long delay since my last log entry.  It's been way too long (2 years?).  Life intervenes sometimes its own priorities.

I had and excellent question from someone who watched one of my videos and so I thought i would share it and my response:

"Just a question? Why would you go through the trouble of designing and have boards built for a learning designed for bread boards? Just courious. I like the video and will continue to follow. I am building the same thing on bread boards and plan on moving to the SAP-2 after."

My response:

"Hi Robert. Thank you for the question. It is a great question! I know, if you're building something for learning and need the flexibility of reworking circuits, etc... And, if you're going to at some point tear it apart and build something else, then bread boarding is a good way to go. I know that Ben Eater promotes bread boarding for learning, because you get to learn the workings of each isolated module and how they fit together as a whole. And, Ben sells the bread board kits which supports his ability to teach these concepts. He's a great educator and I've really enjoyed watching and learning from his videos.

My preference for designing and laying out circuit boards is to take this project in a different direction. For me, I don't totally trust bread boards for large circuits; I prefer soldered connections. As many people have successfully built these things, this is clearly a personal preference on my part. Also, in the process of building my own boards, I'v learned a few things that Ben doesn't cover. For instance, I've had issues with double clocking of my micro code sequencer and weird jitters with the ALU when a certain combination of adding occurs. I've had signal levels pulled down by the way Ben places LEDs on the outputs of the TTL circuitry to the point where the output signal is not read properly by the following input circuit. This is why I later added resistors and LED drivers. And, I think I've suffered from signal cross talk. I found a wonderful explanation of it from a man named Robert Baruch. He has several videos on the subject starting with:

Also, I thought if I were successful with this build, then I could frame it and hang it on the wall as geek art.

And, my end goal is to build a discreet component version of one of my favorite microprocessors from the old days. And so, I'm learning the pitfalls on a simpler machine before I move onto something more complicated.

As a result of these boards, which, by the way, I never got to work because of the aforementioned issues, I'm rethinking how I wish to proceed. Currently, I'm starting with more of a modular approach.... i.e. each register has it's own card that plugs into a back plane such that the LEDs are on the edge of the board. The ALU, for instance would be a self contained module with a back plane, ALU card and some register cards. Is this over kill? Perhaps. But, in the end, it's all about having fun, creating something fun and learning tons in the process. Sometimes I think I enjoy laying out boards (as a zen thing) more than I do with having them do something useful.

I hope this helps answer your question. There are many paths to electronic Nirvana and I wish you well and success on yours. "


zpekic wrote 07/08/2020 at 02:51 point

Great project! Soldering is fun, and often times even works! :-) Modular approach is really great idea too, on that note what if board 1 is a "slice"? With slight change (mostly exposing carry in / out and zero flags a bit differently) one could create the simplest 4-bit computer with 1 board 1 and 1 board 2. Then with adding more board 1s, 8, 12, 16, 24 ... bit computers can be created. For those, different board 2s could be developed using different technologies and approach, maybe even for fun competition. 

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